Diffraction gratings are frequently used in lasers for reflecting back into a laser's resonating cavity only a narrow range of wavelengths of light centered at one particular wavelength. Light energy at this wavelength resonates within the cavity and is emitted through a partially reflective mirror at the other end of the cavity. Examples of such diffraction gratings and various methods of making these diffraction gratings are described in U.S. Pat. Nos. 5,080,465; 5,436,764; and 5,493,393, incorporated herein by reference.
Typically, a master diffraction grating is first manufactured. This master grating is then used to form many replica gratings. Each of these replica gratings may then be used as a master grating for forming other replica gratings. Accordingly, compared to the master, the replicas may be made inexpensively.
As described in the '465 patent, a master grating may be formed by depositing aluminum over a substrate, such as glass. A diamond tool under interferometric control may then be used to etch very closely spaced grooves in the aluminum layer. The separation of the grooves is related to the wavelength of the light to be reflected by the grating and to the narrowness of the range of wavelengths it is required to reflect. In one embodiment, the diamond tool etches on the order of tens of thousands of grooves per inch. The diffraction grating may be ten inches square and one inch thick. Creating a precision master grating by physical etching is, therefore, an extremely time consuming and expensive process.
Once a master grating has been made, replicas of the grating are made in the following process. A release agent, such as described in the '764 patent, is coated on the surface of the master. This is preferably done in a vacuum chamber. A thin (e.g., 1 micron) reflective layer, such as aluminum, is then sputtered or evaporated onto the release layer. The master grating is then removed from the vacuum chamber. Liquid epoxy, such as described in the '465 patent, is then deposited on the aluminum layer, and a glass substrate is then placed on top of the epoxy. After the epoxy is cured, the glass layer, epoxy layer, and aluminum layer are then lifted from the master grating, resulting in a replica of the master grating.
Applicant has discovered that, when the replica is removed from the master grating, imperceptible damage is done to the thin layer of aluminum on the replica.
When the replicated diffraction grating is used as a selective wavelength reflector for generating a laser light beam, the replicated grating is subjected to intense light energy. Changes in the groove shape occur when the grating is exposed to intense light for long periods of time. This causes a loss of reflectance in the desired diffraction order, where some of the optical energy is shifted to other diffraction orders due to large scale groove angle changes, and some of the energy is lost to scatter due to small scale surface distortions of the groove faces.
Up until now, it was believed that the lifetime of such replicas could not be extended.
Applicant has discovered that the aluminum coating deposited during the replication process, typically about 1 micron thick, is subjected to forces during the subsequent take-apart step of the replication process which tend to generate very small fractures in the aluminum coating. These fractures allow small quantities of ultraviolet radiation to leak through to the underlying epoxy when the grating is put into service in an excimer laser. The ultraviolet light which reaches the epoxy causes photodecomposition of the epoxy, releasing gases which cause blisters in the overlying aluminum coating. This blistering greatly increases scatter losses from the reflecting facets of the grating. The ultraviolet light also causes bulk shrinkage of the epoxy, which distorts the original groove shape, causing a loss of reflectivity in the desired diffraction order. These effects severely limit the useful lifetime of the grating, causing unavailability of the equipment in which it is used at relatively frequent intervals.
What is needed is an improved replication process which lengthens the life of the replica gratings.